This invention is related to a dual function stabilizer for organic materials, whether natural or synthetic, which are to be protected against degradation both by ultraviolet ("UV") light and oxygen, hence termed "dual-function". More specifically, the invention is related to an antioxidant ("AO") and UV-stabilizer ("UV-S") the dual function of which is attributable to the combination of a 1,2-ethanediyl-bis-(N.sup.4 -polysubstituted-2-piperazinone) UV-stabilizing moiety and an ester moiety which contributes a surprising level of AO activity even when the ester does not include a hindered phenol (which one would expect to contribute such activity). The term "polysubstituted" refers specifically to a N-containing heterocyclic ring compound such as piperidine, piperazine, or 2-piperazinone in which at least the N-adjacent carbon atoms in the ring are each dialkyl-substituted, or one of the #3 or #5 C atoms, or each of them, is shared by a spiro cycloalkylene substituent. Such polysubstituted compounds belong to a class of compounds referred to as hindered amines which are known to be particularly useful as UV-S compounds in synthetic resinous materials.
As is well known, though numerous AOs and UV-Ss provide a significant measure of protection against degradation due to heat, very few UV-Ss provide appreciable protection against degradation due to oxygen. In particular, the clear instruction for the use of N-containing heterocyclic ring compounds having UV-S activity, is that they be used in admixture with AOs to provide the desired dual function.
For example, U.S. Pat. Nos. 4,167,512 and 4,190,571 to Lai et al disclose that a polysubstituted 2-piperazinone ("PSP") is an excellent UV-S, but teach that an AO be mixed with it to provide an organic material with protection, both against u-v light and oxidation. U.S. Pat. No. 3,899,491 to Ramey et al discloses a polysubstituted piperazine which is esterified with a short chain fatty acid ester (acetate, say) through the N.sup.1 position which is not hindered. With this esterified piperazine, they teach that it is desirable to incorporate into the resin composition, sufficient thermal antioxidants to protect the plastic against thermal and oxidative degradation, and they specify a wide spectrum of phenolic antioxidants suitable for the purpose.
In our U.S. Pat. No. 4,477,665 we taught that a dual function stabilizer can be formed by esterifying an alkanol substituent on the N.sup.1 atom of the PSP ring with a carboxylic acid functional hindered phenol, so as to leave a terminal hindered phenol group. Ignoring the fact that the alkanol substituent is on the unhindered N.sup.1 atom in the '665 PSP, it is found that an analogous reaction does not proceed very satisfactorily with a hydroxyalkyl substituent on the hindered N.sup.4 atom. The ester interchange does. This appears to be a peculiarity of the closely coupled PSP moieties in the bis-PSP compound, which of course, the '665 PSP is not.
Thus, the prior art simply failed to teach the desirability of providing the N-heterocyclic rings of a closely coupled bis-PSP with a hindered phenol terminal group in an ester substituent. Nor did it teach providing the bis-PSP with a long chain fatty acid ester. Most of all, there is no suggestion that this be done through the hindered N.sup.4 position, which is what we have done.
Further, the esterification of a piperazine compound, such as is disclosed in the '491 patent, or a piperidine, is comparatively easy because of its relatively basic nature. By comparison, the esterification of a PSP is not, because the carbonyl (2-one) group in the ring renders it far less basic. Ignoring for the moment, the immediately evident resistance to substitution of the hindered N.sup.4 position, the concept of esterifying the PSP through its hindered N.sup.4 position would not appear readily to be translated into reality. The added complication of esterifying the bis-2-piperazinone compound in which two PSP moieities are closely coupled, thrust the concept even further from ready doability.
Referring further to the '571 patent, it teaches that 1,1'-(1,2-ethanediyl)bis[3,3,5,5-tetramethyl-2-piperazinone] is an excellent UV-S and that the N.sup.4 position of each ring may be hydroxyethylated. Though the u-v stabilization ("UV-S") effectiveness of this bis-compound is not vitiated by hydroxyethylation, the presence of hydroxy groups is generally deemed undesirable because they render the UV-S solvent-extractable, and specifically, water-extractable. For many applications where resistance to extraction with water is not a factor, less than excellent resistance of the hydroxyethylated bis-compound is not a significant drawback; in still other instances, the separate addition of an antioxidant is undesirable. In those instances where water extraction is a factor, and a phenolic antioxidant is difficultly incorporated into the polymer to be stabilized, the foregoing drawbacks become critical.
The concept of coupling a nitrogen-containing heterocyclic ring compound with anything, whether a known heat stabilizer or an AO, in the same molecule, through either N-atom of the piperazine ring was out of favor, as is evident from the teachings of U.S. Pat. No. 4,097,452 to Mayer et al who taught that protection against both heat (not oxygen) and light degradation was better provided by spiro substituents.
In fact, the necessity of combining the UV-S and AO moieties for their respective activities arose through the common happenstance, for one reason or another, of dissatisfacation with stabilized polymers containing an admixture of UV-S and AO. One reason is that the UV-S and AO do not get essentially homogeneously dispersed in the polymer, with the result that one typically bleeds to the surface. For another, the interaction of separate UV-S and AO compounds in a mixture subjected to thermoforming conditions is difficult to predict.
Thus, many polysubstituted piperazinones have a significant level of UV-S activity, but each has one or more drawbacks which makes the one less desirable from a practical and utilitarian point of view, than another having a less objectionable drawback. This reality dictates the unending search, even in the narrow field of piperazinone-based stabilizers, for compounds with better UV-S activity, and results in discarding numerous stabilizers which do not have commercially significant activity.
Having found that we could esterify the N.sup.4 hydroxylated bis-PSP by an ester interchange reaction so as to leave terminal hindered phenol groups on a hindered substituent to generate a macromolecule of bis-PSP, referred to herein as "the esterified bis-PSP", we found that some polymers stabilized with the esterified bis-PSP were prone to gas fade. Gas fade is evidence of sensitivity to nitrogen oxides present in the atmosphere and results in yellowing of the polymer, which in many instances, is unacceptable.
Moreover, the presence of the terminal hindered phenol groups greatly increased the polarity of the macromolecule so that its likelihood of being resistant to extraction with polar solvents such as water, was expected not to be high. As is well known, highly polar compounds dissolve in highly polar solvents (see Organic Chemistry by Morrison and Boyd, pg 31, 3rd ed, Allyn & Bacon, Inc. Boston). We were surprised to find the resistance to water extraction of the esterified bis-PSP from a synthetic resin such as polypropylene, was excellent. But the problem of gas fade persisted. So it was that we sought to substitute the hindered phenol with another moiety which did not subject the macromolecule to gas fade, was able to inculcate it with AO activity, yet have excellent resistance to extraction with solvents, particularly water.
It was in the foregoing factual framework that a solution was discovered to the problem of combining a single bis-PSP moiety having known UV-S activity, with an ester of a hindered phenol known to provide desirable dual function UV-S and AO activity; and, generating a macromolecule with plural repeating units of the bis-PSP esters of other moieties not known to provide AO activity, in such a way that they provide the dual function activity which resists extraction with hot water.
In particular, since there was nothing to suggest that a long chain fatty acid might contribute any AO activity to the bis-PSP macromolecule, we concluded it was unlikely to do so, yet be resistant to solvent extraction with solvents. We made the bis-PSP long chain fatty acid ester terminated macromolecule nevertheless, and happily discovered we were wrong. Moreover, the polarity of the macromolecule did not seem to contribute much to extractability, provided there was sufficient length to the macromolecule. This requirement of length, led to the synthesis of dual function bis-PSP oligomers having plural esterified bis-PSP repeating units, referred to herein as "esterified bis-PSP oligomers", by ester interchange. We found that a variety of ester moieties alternating with the bis-PSP in the esterified bis-PSP oligomers show desirable dual function activity.